The present invention relates to a method for the digital control of a universal motor, notably for domestic use, and more particularly such a method comprising the steps consisting in
measuring the speed of rotation of the said motor,
determining the difference between the measured speed and a reference speed, and
controlling the said motor according to the said difference.
Universal motors are single-phase motors with a series excitation commutator, and are frequently used in the field of domestic electrical equipment, notably for driving the drums of washing or drying machines.
The regulation of the speed of rotation of a universal motor involves a tachometric generator coupled to this motor in order to supply as an output an electrical signal whose frequency is proportional to the speed, and a supply triac whose triggering angle determines the mean voltage applied to the motor windings. A control law makes it possible to adapt the triggering angle according to the speed.
Digital control methods have already been proposed. Currently, the law used is very simple since a triggering angle difference proportional to the speed difference is adopted:
xcex1n+1=xcex1n+Axc2x7xcex94xcfx89xe2x80x83xe2x80x83(1)
where
xcex1i is the triggering angle of the triac at time ti 
xcex94xcfx89 is the difference between the speed measured by the tachometric generator and the reference speed, and
A is a proportionality coefficient.
In addition, provision is generally made for limiting the current in the motor. To this end, a shunt makes it possible to measure this current.
Although generally giving satisfaction, this method has a certain number of drawbacks.
Firstly, it provides no access to the physical quantities other than those which are directly measured, namely essentially the speed of rotation and possibly the current in the motor. The known method is therefore relatively incomplete.
In addition, knowledge of the current requires, in addition to the presence of the shunt, knowledge of an amplifier and current to voltage converter. The result is a complication of the circuit and consequently an increase in costs. In particular, it cannot supply the true effective current without fairly powerful calculation means.
However, this knowledge of the current is particularly useful in certain applications, independently of the need to limit it, and notably in the case of the driving of the drum of a washing machine. This is because, in the case of a universal motor, the torque is directly a function of this current. And knowing the torque gives access to the load, on the assumption that the motor follows the acceleration inputs.
The load is a quantity, knowledge of which makes it possible to regulate the speed in an optimum fashion in the aforementioned case of the driving of the drum of a washing machine. This is because the effectiveness of the washing is to a major extent a function of the relative speeds of the washing and the water contained in the drum. However, when the load is low, the washing falls by itself in the washing water during its rotation. The control law can therefore advantageously be adapted to the machine load.
The present invention aims to mitigate the drawbacks set out above.
More particularly, the invention aims to provide a digital control method for a universal motor, notably for domestic use, which dispenses with the use of the shunt of the prior art.
The invention also aims to provide such a control method which makes it possible to take account of the motor load without requiring any current measurement.
In addition, the invention aims to provide such a control method which makes it possible, in the case of a washing or drying machine, to take account of the imbalance created by an irregular arrangement of the washing.
To this end, the object of the invention is a method for the digital control of a universal motor, notably for domestic use, comprising the steps consisting in
measuring the speed of rotation of the said motor,
determining the difference between the measured speed and a reference speed, and
controlling the said motor according to the said difference,
characterised by the fact that it also comprises a step of estimating at least one of the quantities including the resistive torque (C) and the current (i) in the motor windings.
It will be seen that it is possible to deduce the motor load from the resistive torque (C), which is particularly advantageous in the case of a domestic electrical appliance in which this parameter is generally unknown to the user.
The method according to the invention can also comprise the estimation of at least the moment of inertia or the coefficient of viscous friction.
It is also possible to make provision for the estimation of the resistive torque to comprise the estimation of the sum of a substantially constant mean torque and a sinusoidal imbalance component.
It is then possible, in the case of a washer, to take account of an irregular arrangement of the washing in the drum in order to obtain a more even speed of rotation of the latter.
Also in a particular embodiment, the method of the invention can comprise the step consisting in determining an estimation, possibly optimum, of the speed of rotation of the motor.
Also in a particular embodiment, it may comprise the step consisting in determining an estimation of the motor load.
Such estimations make it possible in particular to provide an optimum control taking account also of the load and operating temperature of the motor.
In particular, the method according to the invention can comprise the step consisting in determining the cycle ratio of the motor operating cycle. Cycle ratio of the motor operating cycle means here the ratio between the actual operating time and the total time of the cycle, including the time during which the motor is stopped.
In a first embodiment of the invention, the motor speed regulation is effected by any known means. It is possible in particular to determine the triggering angle of the triac by a formula such as formula (1).
It is possible on the other hand, in another embodiment of the invention, to include the aforementioned estimations in the regulation loop. The control voltage to be applied to the motor is then calculated according to these estimations, and the triggering angle of the triac is derived from this calculated voltage.
Consequently, in the method according to this embodiment of the invention, there is determined, not directly and empirically the triggering angle of the triac, but the voltage to be applied to the motor input. Being in possession of this voltage, it is possible to derive therefrom, in addition to the triggering angle, various electrical and mechanical operating parameters, and thus to optimise control.